Spatial Division Multiple Access (SDMA) is a method of multiplexing several signal streams, each one targeted to a different destination, simultaneously, by utilizing multiple transmit antennas. An SDMA channel access method may enable the use of the same frequency at the same time to communicate with several stations (STAs, also referred to as nodes, receivers, beamformees or users) located in different places. For example, in Multi-User Multiple-Input-Multiple-Output (MU-MIMO) communication systems, a multi-antenna beamformer, which is typically the SDMA Access Point (AP), may transmit simultaneously to a group of STAs, using SDMA technique. A group of STAs that the beamformer transmits to in a single transmission is referred to as a MU group.
The IEEE 802.11ac standard, which supports downlink (DL) MU-MIMO technology, defines the capability for a beamformer, typically an AP, to transmit several simultaneous streams to several stations in a network in the downlink direction. As used herein, the downlink direction is used to mean the direction from a beamformer to one or more stations receiving the beamformed signals. Also, as used herein, MU-MIMO transmission refers to transmission of several simultaneous streams to several stations in a network in the downlink direction.
MU-MIMO transmission requires a scheme for MU group management. For example, according to a prior art scheme operable under the IEEE 802.11 ac standard, a single beamformer, which is typically the AP, may group users into MU groups using the unicast MU group assignment frame. Thus only a single predetermined and unchangeable entity in a network is permitted to perform tasks related to MU group management, such as grouping users into MU groups, changing these MU groups and notifying the STAs of their group assignments.